In recent years, a high-power fiber laser using a rare-earth-doped fiber has been attracting public attention. The high-power fiber laser is configured such that, while pump light and signal light are propagating through a fiber, the signal light is amplified by the pump light. The high-power fiber laser has advantages such as ease of cooling and reduction in size.
In such a high-power fiber laser, in order to suppress nonlinear effects, a fiber having a large mode field diameter (MFD) is used. However, if the mode field diameter is simply increased, higher-order modes may propagate. The propagation of the higher-order modes may cause deterioration of beam quality. For this reason, there is a need for a method that is capable of suppressing propagation of higher-order modes.
Non-Patent Documents 1 and 2 are examples of related art for suppressing propagation of higher-order modes.
These documents disclose techniques in which higher-order modes propagating through a core having a refractive index larger than a cladding are coupled to a mode propagating into an area around the core with a raised refractive index, thereby suppressing propagation of higher-order modes through the core.
[Non-Patent Document 1] “Design of solid and microstructure fibers for suppression of higher-order modes,” Optics express, Vol. 13, No. 9, pp. 3477, 2005
[Non-Patent Document 2] “Design of microstructured single-mode fiber combining large mode area and high rare earth ion concentration,” Optics express, Vol. 14, No. 7, pp. 2994, 2006